Growth Of Single Crystal Fibers Research Articles

Dynamics of Er3+:YAG thermal radiation spectra near solid-melt interface at single crystal fiber growth process

The evolution of the Er3+:YAG thermal radiation spectra at heating up to the melting point was investigated in the visible range when growing a crystal fiber by the Laser-Heated Pedestal Growth (LHPG) method. The Er3+:YAG melt of about 1 mm thickness radiates like a black body in several spectral bands sometimes different from the crystal absorption lines. The real melt temperature in the middle of the growth zone was estimated as 2620 ± 15 K. The spectral profile of Er3+:YAG emissivity in the visible range for a melt layer of about L ≈ 1 mm at 2620 K is shown in the Figure.

Temperature and emissivity measurements at the sapphire single crystal fiber growth process

We present a new method for evaluation the absorption coefficient of the crystal melt around the phase transition zone for the spectral range of semitransparency. The emissivity distribution across the crystallization front of the sapphire crystal fiber was measured at the quasi-stationary laser heated pedestal growth (LHPG) process (Fejer et al., 1984; Feigelson, 1986) and the data for solid state, melt and phase transition zone (melt-solid interface) were obtained. The sapphire melt absorption coefficient was estimated to be 14 ± 2 cm−1 in the spectral range 1–1.4 μm around the melt point. It is consistent with data, obtained by different other methods. This method can be applied to determine the absorption coefficient for other materials.

Growth and spectroscopic characterizations: properties of Nd:LiLa(MoO4)2single crystal fibers

In this work single crystal fiber growth by the micro-pulling-down technique and the structural and spectroscopic characterizations of undoped and Nd3+-doped double molybdate LiLa(MoO4)2 are reported. The optimization of the growth conditions was studied in order to avoid facet formation. The grown fibers are free of scattering centers and the real dopant concentrations were confirmed to be very close to the nominal ones with a quite uniform distribution along the fibers length. The crystallographic structure concerning the scheelite tetragonal symmetries was re-evaluated. Time-resolved fluorescence spectroscopy was applied to the evaluation of some optical properties. Selective laser excitation of the 4F3/2 at 875 nm and 4F5/2 at 805 nm has established that the energy levels above the 4F3/2 level are entirely quenched by multiphonon emission in Nd:LiLa(MoO4)2 crystal fiber. The potential laser gain of the system was determined using a numerical solution of the rate equations system for the 805 nm continuous pumping regime, which shows the maximum gain of the laser emission at 1.064 μm for using a Nd3+-doping of 5 mol%.

Growth and characterization of LiYF4: Er+3 fibers

The growth of single crystal fibers of LiYF4 (YLF) doped with Er3+ was studied through the resistive micro-pulling-down (μ-PD) technique. The growth chamber was modified to achieve vacuum of approximately 10−7torr. Single crystal fibers of YLF: Er (1, 10 and 20 mol %), with 0.7 mm of diameter and up to 120mm of length were grown. The use of a static atmosphere was found to be inappropriate for elimination of spurious contamination from growth environment for this experimental setup.

Nd : YVO 4 single crystal fiber growth by the LHPG method

Nd : YVO 4 single-crystal fibers of various compositions were grown by the laser-heated pedestal growth (LHPG) method. During the crystal growth, a thermal imaging radiometer was used to observe the thermal radiation emitted by the melt, ceramics and crystals. Parameters of the LHPG method, such as powder compositions, ceramic process, and crystal-growth conditions are discussed. With the higher pulling rates during the LHPG process and proper ceramics-production control, the decomposition of V 2O 5 could be reduced and a-axis Nd : YVO 4 crystal fibers were grown. Lattice constants of the grown crystal fibers were measured.

Barium sodium niobate single crystal fiber growth by the micro-pulling down (μ-PD) technique

Growth of single crystal fibers from the congruently melting composition of barium sodium niobate Ba 2.065Na 0.81Nb 5.01O 15 melt using a micro-pulling-down system has been demonstrated. This method avoids some problems associated with conventional melt techniques such as the Czochralski method. Neither cracks nor any other kind of inhomogeneity could be detected in the crystals, which are therefore suitable for nonlinear optical applications. Results of coupled differential thermal and thermogravimetric analysis indicated that the melting point of the starting mixture was 1453°C. No phase transitions other than melting and solidification were detected.

YVO 4 single-crystal fiber growth by the LHPG method

In this study, we use the laser-heated pedestal growth (LHPG) method to grow yttrium orthovanadate (YVO 4) single-crystal fibers. The stoichiometric and nonstoichiometric ceramic rods of YVO 4 are obtained and then melted by a CO 2 laser. During the YVO 4 growth process, we use a thermal imaging radiometer to observe the thermal radiation emitted by the melt, ceramics and crystals. With the higher pulling rates during the LHPG method process and proper thermal imaging process control, the dissociation of V 2O 5 is reduced and we obtain transparent a-axis YVO 4 crystal fibers of length 40 mm. We also examine the lattice parameters of the grown crystal fibers.

Growth of single crystal fibers for 1.3 μm optical amplifiers by the laser-heated pedestal growth method

We describe the use of the laser-heated pedestal growth (LHPG) technique to prepare single-crystal fibers of potential solid-state laser and optical amplifier materials. Compared with other crystal growth approaches, including Czochralski and flux growth, the LHPG approach, which has very large thermal gradients at the growth interface, has fewer dislocations, resulting in crystals with improved optical quality. We discuss the use of the LHPG technique for the growth of spinel MgAl 2O 4:Ni 2+ crystals. Such crystals are interesting as candidate materials for a 1.3 μm optical amplifier.

Modeling the time-dependent growth of single-crystal fibers

A model is developed to simulate time-dependent behavior in laser-heated miniature pedestal growth (LHPG) of single-crystal fibers. This process is meniscus controlled since the diameter of the growing fiber changes in response to variations between the equilibrium and the instantaneous contact angle at the melt/solidifying-solid interface. One of the operational problems associated with the LHPG technique is controlling diameter fluctuations in the growing fiber due to process perturbations. Smooth source feed and fiber pull rates, steady laser-heat input, and suppression of mechanical vibrations are necessary to achieve a constant fiber diameter. We derive a system of time-dependent, one-dimensional equations of motion and heat transfer as the leading order equations in an asymptotic expansion based upon the melt slenderness ratio (fiber radius/melt height). This system is solved numerically to predict the thermal profiles, mean positions of the melting and solidifying fronts, melt/gas interface, melt volume, and fiber diameter. System response to perturbations in the pull rates, thermal environment, and gravitational field is investigated.

The Formation and Stability of the Molten Zone in Single-Crystal Fiber Growth by the LHPG Method

The Formation and Stability of the Molten Zone in Single-Crystal Fiber Growth by the LHPG Method

Novel implementation of laser heated pedestal growth for the rapid drawing of sapphire fibers

A new version of laser heated pedestal growth system is described which is amenable for the rapid growth of single crystal fibers. The system incorporates a Gaussian reflector to reshape the radial intensity profile of the CO2 laser beam. When He is used as the growth atmosphere, high optical quality sapphire fibers can be produced at rates as high as 2 cm/min with this apparatus.

Factors affecting the growth of single crystal fibers of the superconductor Bi 2Sr 2CaCu 2O 8

While short sections of single crystal fibers of the 85 K bismuth compound superconductor can be produced by the miniaturized laser-heated float zone process, it is much more difficult to produce extended lengths without the introduction of additional grains. Part of the problem is related to the peritectic nature of the material. Detailed studies have been performed to determine the influence of growth rate, seeding, and laser power (melt temperature) fluctuations on grain structure and second phase inclusions in fibers grown from a Bi 2.1Sr 1.8Ca 1.1Cu 2O x starting material. It was found that as the fiber diameter decreases, the maximum allowable growth rate increases due to an increase in the axial temperature gradient. Thermal fluctuations were shown to introduce CaO inclusions. Grain structure is not altered unless the fluctuations produce a large amplitude diameter variation. Several seeding techniques are discussed.

Growth of Single Crystal Fibers

Single-crystal fibers represent perhaps one of the most underexplored areas of material science. Despite some early interest and excitement 30 years ago, the lack of immediate applications led to a decline in research activity, and until recently, little was known about their growth behavior and properties. During the past few years single-crystal fiber research has been revived and preliminary results have stimulated considerable interest in the scientific community.At present, the most extensive and broadly based single-crystal fiber program is at Stanford University, where the focus has been on studying fibers for optical applications. This program emerged from a desire to combine the light-guiding properties of fiber geometries with the unique physical properties of crystalline materials, as was done during the last decade for glass fiber applications. Such materials could lead to a range of novel devices with higher efficiencies than possible in bulk crystals. But single-crystal fiber growth technology may have an even broader applicability. While single-crystal fibers of semiconductor, superconductor, and high strength materials could play an important role in future device applications, an important current use for the single-crystal fiber growth technology is in preparing single crystals of a wide variety of materials for property evaluation. Fiber crystals may have much higher crystalline perfection than bulk crystals of the same composition, making them useful for studying the intrinsic properties of a material and for improving device performance. Other areas of interest which have been identified include metastable phase formation and the growth of oriented ferroic domain and multiphase structures.

Growth of tetragonal BaTiO3 single crystal fibers

With the increasing use of optical fibers in the telecommunication network, there is need for fiber geometry compatible optical devices such as optical amplifiers, switches, couplers, and isolators. These active devices are based on field-dependent material properties, such as electrooptic and magneto-optic effects, which are stronger in single crystal than in amorphous materials. Single crystal fibers can be grown by the laser heated pedestal growth (LHPG) technique. In this paper we report the growth of single crystal fibers of ferroelectric barium titanate from sintered ceramic rods of stoichiometric barium titanate. Barium titanate is one of the most extensively investigated ferroelectric materials. However, its growth from stoichiometric melt always results in its hexagonal nonferroelectric phase. Using LHPG, single crystal strontium titanate seed, and sintered ceramic barium titanate rods, we have succeeded in growing single crystal fibers (∼ 100 μ m diameter) of pure barium titanate with tetragonal (ferroelectric) crystal structure. This paper discusses growth and characterization of these fibers.